SEATTLE — Legal marijuana possession becomes a reality under Washington state law on Thursday, and some people planned to celebrate the new law by breaking it.

Voters in Washington and Colorado last month made those the first states to decriminalize and regulate the recreational use of marijuana. Washington’s law takes effect Thursday and allows adults to have up to an ounce of pot — but it bans public use of marijuana, which is punishable by a fine, just like drinking in public.

Nevertheless, some people planned to gather at 12 a.m. Thursday to smoke beneath Seattle’s Space Needle. Others planned a midnight party outside the Seattle headquarters of Hempfest, the 21-year-old festival that attracts tens of thousands of pot fans every summer.

Seattle Police spokesman Sgt. Sean Whitcomb said he doesn’t expect officers to write many tickets to the celebrants. Thanks to a 2003 law, marijuana enforcement remains the department’s lowest priority. Even before Initiative 502 passed on Nov. 6, police rarely busted people at Hempfest, despite widespread pot use, and the city attorney here doesn’t prosecute people for having small amounts of marijuana.

Washington’s new law decriminalizes possession of up to an ounce for those over 21, but for now selling marijuana remains illegal. I-502 gives the state a year to come up with a system of state-licensed growers, processors and retail stores, with the marijuana taxed 25 percent at each stage. Analysts have estimated that a legal pot market could bring Washington hundreds of millions of dollars a year in new tax revenue for schools, health care and basic government functions.

But marijuana remains illegal under federal law. That means federal agents can still arrest people for it, and it’s banned from federal properties.